An adaptive multichannel radiometer designed to detect frequency-hopping (FH) signals in complex signal environments is presented. Each channel updates its hop threshold to reflect the current environment and excise any persistent hop activity inconsistent with an FH signal from subsequent processing. This strategy allows the receiver to discriminate FH signals from any random noise or interference activity with relatively small degradations as compared to operation in stationary additive white Gaussian noise. Two data collection schemes are considered for the proposed receiver, both of which attempt overall decisions using fixed-length blocks of data. In the first scheme, block detection, successive decisions are based on consecutive, nonoverlapping blocks of data, whereas in the second, block-sequential detection, decisions are made each time a new datum is collected. The block-sequential scheme is shown to offer greatly reduced average signal detection times and, thus, is the preferred approach. >